1. Field of the Invention
The present invention generally relates to a method and apparatus for reducing the effect of unwanted noise in seismic data caused by nearby seismic or other noise-producing activity. More particularly, the present invention relates to reducing noise in marine seismic data caused by seismic activity from nearby seismic ships. Still more particularly, the present invention relates to eliminating noise from nearby seismic ships during real or near real-time data acquisition or in subsequent processing.
2. Background of the Invention
The field of seismology focuses on the use of artificially generated elastic waves to locate mineral deposits such as hydrocarbons, ores, water, and geothermal reservoirs. Seismology also is used for archaeological purposes and to obtain geological information for engineering. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data, can provide information about the structure and distribution of rock types and contents.
Most oil companies rely on seismic interpretation to select sites for drilling exploratory oil wells. Despite the fact that the seismic data is used to map geological structures rather than find petroleum directly, the gathering of seismic data has become a vital part of selecting the site of exploratory and development wells. Experience has shown that the use of seismic data greatly improves the likelihood of a successful venture.
Seismic data acquisition is routinely performed both on land and at sea. At sea, a seismic ship deploys a streamer or cable behind the ship as the ship moves forward. The streamer includes multiple receivers in a configuration generally as shown in FIG. 1. Streamer 110 trails behind ship 100, which moves generally in the direction of the arrow 101. The streamer includes a plurality of receivers 114. As shown, a source 112 is also towed behind ship 100. Source 112 and receivers 114 typically deploy below the surface of the ocean 70. Streamer 110 typically includes electrical or fiber-optic cabling for interconnecting receivers 114 and seismic equipment on ship 100. Streamers are usually constructed in sections 25 to 100 meters in length and include groups of up to 35 or more uniformly spaced receivers. The streamers may be several miles long and often a seismic ship trails multiple streamers to increase the amount of seismic data collected. Seismic data acquisition involving a single streamer including a single source and multiple receivers is referred to as two-dimensional ("2D") seismic data acquisition. The use of multiple streamers and/or multiple sources is referred to as three-dimensional ("3D") seismic data acquisition. Data is digitized via electronic modules located near the receivers 114 and then the data is transmitted to the ship 100 through the cabling at rates of 7 (or more) million bits of data per second. Processing equipment aboard the ship controls operation of the trailing source and receivers and performs initial processing on the acquired data. The large volume of data acquired usually requires further processing in land-based computing centers after the seismic survey has completed.
Seismic techniques estimate the distance between the ocean surface 70 and subsurface structures, such a structure 60 which lies below the ocean floor 63. By estimating the distance to a subsurface structure, the geometry or topography of the structure can be determined. Certain topographical features are indicative of oil and/or gas reservoirs. To determine the distance to subsurface structure 60, source 112 emits seismic waves 115, which reflect off subsurface structure 60. The reflected waves are sensed by receivers 114. By determining the length of time that the seismic waves 115 took to travel from source 112 to subsurface structure 60, and to receivers 114, an estimate of the distance to subsurface structure 60 can be obtained.
The receivers used in marine seismology are commonly referred to as hydrophones, or marine pressure phones, and are usually constructed using a piezoelectric transducer. Synthetic piezoelectric materials, such as barium zirconate, barium titanate, or lead mataniobate, are generally used. A sheet of piezoelectric material develops a voltage difference between opposite faces when subjected to mechanical bending. Thin electroplating on these surfaces allows an electrical connection to be made to the device so that this voltage can be measured. The voltage is proportional to the amount of mechanical bending or pressure change experienced by the receiver resulting from seismic energy propagating through the water. Various configuration of hydrophones are used such as disk hydrophones and cylindrical hydrophones.
Two types of seismic sources are used to generate seismic waves for the seismic measurements. The first source type comprises an impulsive source which generates a high-energy, short time duration impulse. The time between emitting the impulse from the source and detecting the reflected impulse by a receiver is used to determine the distance to the subsurface structure under investigation. A second type of source generates lower magnitude, vibratory energy. The measurement technique that uses such sources is referred to as the marine vibratory seismic ("MVS") technique. Rather than imparting a high magnitude pressure pulse into the ocean in a very short time period, vibratory sources emit lower amplitude pressure waves over a time typically between 5 and 8 seconds, although longer time periods are also possible. Further, the frequency of the vibrating source varies from about 5 to 150 Hz, although the specific low and high frequencies differ from system to system. The frequency of the source may vary linearly or non-linearly with respect to time. The frequency variations are commonly called a "frequency sweep." The frequency sweep may thus be between 5 and 150 Hz and 5 to 7 seconds in duration. The magnitude of the seismic wave oscillations may vary or remain at a constant amplitude, but generally are much lower than the magnitude of impulsive sources.
The amount of data collected in a typical seismic survey can be voluminous. For example, a typical seismic survey may involve the mapping of a 1000 square mile region of the ocean by a 3D seismic ship trailing six or eight streamers. Each streamer may have 400 or 500 receivers attached to it. For each seismic measurement, 6-8 seconds of data (referred to as a "trace") is acquired and stored on magnetic tape on-board the ship. To completely map the survey area, which may require several weeks, one billion traces, or more, may be acquired and stored on tape. The traces are stored as "shot records" on the tape with a shot record representing the traces from all of the receivers from a single shot pulse from a source. This volume of data necessitates the use of thousands of magnetic tapes which are manually loaded into storage bins in the ship initially and then automatically accessed by specialized equipment on-board the ship during the survey. Because of the enormous volume of data acquired during a typical survey, improved techniques for efficiently processing the data are needed.
Some areas of the world are heavily explored so that several seismic ships, working for related or unrelated operators, may be conducting seismic surveys at the same time and in relatively close proximity to one another (within 50 miles or so). The receivers typically used in seismic streamer cables are highly sensitive as well as omnidirectional (sensitive to signals travelling from any direction). Virtually any sound that passes through the location of the receiver is detected by the receiver. Accordingly, the receivers respond not only to an impulse or "shock pulse" generated by their own ship, but may also respond to shots generated by another ship in the vicinity.
FIG. 2, for example, illustrates three seismic ships 20, 30, and 40 in the same general area of the ocean. As shown, seismic signals 31 and 41 generated by ships 30 and 40, respectively, propagate through the water to the receivers 114 on ship 20 as well as reflect off subsurface structures. To ships 30 and 40 their seismic signals 31, 41 represent desirable signals, but to ship 20 those signals represent undesirable noise. Additionally, there are other types of noise external to the seismic acquisition system of ship 20 that affect marine data acquisition. Examples of such noise include weather noise and "cultural" noise from rigs and shipping. All of these noises affect the cost and quality of seismic data by necessitating a relaxation in the specification for data quality control, by requiring that the data be re-acquired (re-shot), or requiring "time sharing" during which closely positioned seismic ships take turns acquiring seismic data to minimize noise on the seismic signals detected by each ship's seismic system. It thus is highly desirable to remove, or at least minimize, the noise present in a seismic signal that is generated from such external sources.
Several algorithms have been suggested for noise reduction. Some of these methods involve a process called "stacking" in which multiple traces are added together or otherwise combined into a single trace. In robust stacking methods high amplitude samples are discarded in the stacking process itself. Trace weighting methods are similar in approach to robust stacking, except that the traces are inversely weighted prior to stacking rather than selectively eliminated during stacking.
Adaptive-predictive methods, such as f-x deconvolution, reduce random noise by predicting coherent events. Since interference is random in common offset or CDP domains, random zones of high amplitude noise can be replaced on a per-sample basis with the surrounding acceptable data. Moveout filtering specifically targets coherent events. This method includes the well-known f-k filter and .tau.-p filter which depend on the signal and interference having different (and separable) "dips" in the common shot domain.
Trace editing techniques attempt to separate high amplitude zones from the surrounding signal. These zones then are either weighted down, blanked or replaced with neighboring data using an interpolation scheme. The present invention relates to an improvement in trace editing methodology.
Conventional trace editing methods generally require (1) all of the seismic data to be collected, (2) sorting the traces into an appropriate "domain," and then (3) removing the noise from the sorted traces. Conventional shot domain methods, while not requiring sorting, do not respond well to inherent variations in signal levels. In a 2D acquisition system, sorting the traces necessarily involves considerable time and processing power because of the volume of data. This processing usually occurs at a processing facility after the seismic survey has been completed. The processing burden is exacerbated in a 3D acquisition system which may involve sorting a billion traces or more. One of the most costly aspects of seismic data-processing involves playing back the traces from the magnetic tapes and storing the processed information back on tape. The processing of tens of thousands of magnetic tapes requires specialized equipment operating over a relatively long period of time and is extremely costly.
Because full seismic data processing is generally considered too expensive a task to be performed on the ship at sea, the seismic operators of the ship have little assurance regarding the quality of the data that they have collected. Quality control "stacks" which are generated on board typically are insufficient to properly evaluate the data degradation. It is not until after the survey has completed and the tapes are processed that the operators will know whether the data is good or not. Often times only select portions of the data may be infected with noise or otherwise corrupted. Reshooting bad sections of a seismic line generally is not economically feasible once the ship has left the survey area. It would be highly advantageous to be able to evaluate the data at sea in real-time, or near real-time, to be able to determine the quality of the acquired data while the data is being acquired. Thus, if it is determined that a particular part of a line needs to be re-shot, those records can be re-shot while the ship is still in the general area. Such real-time data-processing capability would make it economically feasible to re-shoot records when required, thereby increasing the quality of the data.
Accordingly, an improved method of seismic data processing is needed to solve the problems noted above. Such a method preferably could be performed in near-real or real-time while the data is being acquired. Further, the processing method would preferably lower the cost required to process the data. Despite the advantages such a seismic processing system would offer, to date no such system is known to exist.